This invention relates to fuel cells and fuel cell stacks particularly those that employ molten alkali metal carbonate electrolyte.
Molten carbonate fuel cells and stacks of such cells are well known and described in various prior publications and patents. For example, U.S. Pat. No. 4,478,776 to Maricle et al. and U.S. Pat. No. 4,411,968 to Reiser et al. illustrate typical fuel cells and stacks of such cells. Porous, sintered nickel-chromium anodes and porous nickel oxide cathodes are disposed on opposite major surfaces of a porous electrolyte matrix. A matrix of such as lithium aluminate (LiAlO.sub.2), or other inert ceramic is filled with molten alkali metal carbonate electrolyte, (e.g. Li.sub.2 CO.sub.3 /K.sub.2 CO.sub.3) in each cell in a fuel cell stack. Stacks with several hundred fuel cells are contemplated in a typical power supply.
The management of electrolyte within a stack of molten carbonate fuel cells has become a major developmental issue. Molten carbonate electrolyte has high surface tension and forms extensive films at the face surfaces of the stack. Although the exact mechanism is not completely understood, it is believed that an electrolytic pumping action results from electrochemical reactions driven by the stack voltage. This pumping action can cause electrolyte migration along the stack face and through the manifold seals which extend from the positive to the negative end of the stack along the edges of the individual cells. The edges of the individual cells when taken together comprise the stack surface. The result is to dry out the cells at the positive end and flood the cells at the negative end of the stack. In addition, corrosion products can be transported with the electrolyte and deposited at the negative end of the stack. These effects may result in electronic shorts and other conditions that can severely impair the stack performance.
In a fuel cell stack, the electrolyte filled matrix typically will extend to the edge of the stack at all faces to provide a good seal between reactant gases. Unfortunately, this provides a path for molten electrolyte to migrate to the stack surfaces where the manifold seals are located. At the stack surfaces (two of the four), exposed to oxidant gas, this problem is acerbated by the electrochemical pumping of electrolyte through the edge of the matrix. The pumping can be caused by oxygen reduction on exposed metal surfaces in accordance with the reaction: EQU O.sub.2 +2CO.sub.2 +4e.sup.- .fwdarw.2CO.sub.3.sup.-2 1
Hydrogen oxidation at the anode side of the cell can occur by reaction with carbonate: EQU 2H.sub.2 +2CO.sub.3.sup.-2 .fwdarw.2H.sub.2 O+2CO.sub.2 +4e.sup.- 2
These reactions can result in the accumulation of molten carbonate on the air side surfaces of the stack in excess of the amount that would accumulate due only to capillary wicking.
Various attempts have been made to reduce this electrolyte migration. However, such attempts have not proven completely successful but have resulted in unwanted complication of the cell and stack structure.
Therefore, in view of the above it is an object of the present invention to provide an improved molten carbonate fuel cell to limit electrolyte migration from the central body to the edge of the cell.
It is also an object to provide a fuel cell that limits electrolyte migration while providing adequate seals between anode and cathode chambers.
It is a further object of the invention to provide a fuel cell stack with limited electrolyte migration along the manifold seals and stack face surfaces.
It is likewise an object to provide a fuel cell stack with restrained electrolyte pumping to the oxidant face of the fuel cell stack.